Building a DLink DSL-502T Based IGate - The TNC

In addition to configuring the DLink DSL-502T routers for the IGate project we also needed a TNC to connect between the radio and the serial port.

Our TNC selection was based on the following criteria:

  • The TNC board was to be installed inside the DSL-502T case
  • The DSL-502T serial port is used during the powerup sequence to display boot messages and will halt the boot process if it receives any characters so the ability to change the TNC code (or hardware) to not send or receive any data during the boot sequence was critical
  • Low cost
  • Accurate decoding
  • Monitor mode compatible with APRSD

We found the ATMega8 based TNC design from Henry Carl Ott (N2RVQ) which at first glance addressed the requirements and a prototype was built and tested.

In comparative testing of the N2RVQ reference design against an MFJ-1270C TNC using the WA8LMF APRS Test CD we found almost identical performance.  There were a small number of differences in the decode... sometimes the MFJ got packets the ATMega8 didn't and vice versa.

The source code for the ATMega8 TNC is very well written and was easily modified to handle the power on delay required for the DSL-502T boot process.

Long term performance testing of the ATMega8 TNC / DSL-502T / APRSD combination was performed with assistance for Ian ZL1AOX at his QTH overlooking Auckland.  The location of Ian's QTH provided excellent reception over the Auckland area and ensured plenty of traffic was gated through to the APRS-IS network.


After thirty days of continuous operation with no system errors or crashes we were happy enough with the design for Ian ZL1VFO to design a printed circuit board for the ATMega8 TNC that would fit into the DSL-502T case.

TNC Components

Because the DSL-502T would not be used as an ADSL router the telephone interface components were removed from the PCB providing space for the TNC to be mounted on the serial port pin header.

The next image shows the PCB before and after removing the telephone interface components.

DSL502T PCB Comparison

The final result with the TNC mounted on the serial port pin header is shown below.  +12V power for the TNC and available on the radio connector is provided by the red wire.  This connects to the DSL-502T PCB after the diode bridge rectifier and filtering capacitors. 

DSL502T and installed TNC

The current plan is to deploy many DSL-502T/TNC Igates around Auckland and the north Waikato to improve APRS-IS coverage.

 

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Contact Andrew Quinn

jaquinn@ihug.co.nz http://twitter.com/jaquinn